CDK9 and cancer: CDK9 inhibitors can inhibit the kinase activity and phosphorylation of CDK9, thereby preventing P-TEFb-mediated activation of RNA Pol II, and inhibiting gene transcription of many anti-apoptotic proteins.312 Although a large amount of data indicates that CDK9 is a promising target for cancer treatment, the development of highly selective CDK9 inhibitors is still a huge challenge.